Soil Stabilization

SOIL STABILIZATION/GROUND IMPROVEMENT © CIVIL ARCHIVE

1. GENERAL • The soil at a construction site may not always be totally suitable for supporting structures such as buildings, bridges, highways, and dams. •

For example, in granular soil deposits, the in situ soil may be very loose and indicate a large elastic settlement. In such a case, the soil needs to be densified to increase its unit weight and thus its shear strength.

• Improving in situ soils by using additives is usually referred to as stabilization. •

The improvement done by using any kind of materials is generally said to be ground improvement.

2.PURPOSE OF GROUND IMPROVEMENT/SOIL STABILIZATION • Reduce the settlement of structures . • Improve the shear strength of soil and thus increase the bearing capacity of shallow foundations . • Increase the factor of safety against possible slope failure of embankments and earth dams . • Reduce the shrinkage and swelling of soils.

3.GROUND IMPROVEMENT TECHNIQUES 3.1 VIBROFLOTATION

• Vibroflotation is a technique developed in Germany in the 1930s for in-situ densification of thick layers of loose granular soil deposits. • The process involves the use of a vibroflot (called the vibrating unit), as shown in Figure 1

FIG.1: VIBROFLOTATION

3.2 BLASTING

• Blasting is a technique that has been used successfully in many projects (Mitchell, 1970) for the densification of granular soils. • The process involves the detonation of explosive charges such as 60% dynamite at a certain depth below the ground surface in saturated soil. • Compaction (up to a relative density of about 80%) up to a depth of about 18 m over a large area can easily be achieved by using this process. • . The sphere of influence of compaction by a 60% dynamite charge can be given as (Mitchell, 1970): • follows   Where r

→ Sphere of influence

Wex → Weight of explosive → [60% dynamite] C → 0.0122 when Wex is in kg and r is in m

3.3 JET GROUTING • Jet grouting is a soil stabilization process whereby cement slurry in injected into soil at a high velocity to form a soil–concrete matrix. • Three basic systems of jet grouting have been developed—single, double, and triple rod systems. • In all cases, hydraulic rotary drilling is used to reach the design depth at which the soil has to be stabilized. •

The following figure (a) shows the single rod system in which a cement slurry is injected at a high velocity to form a soil–cement matrix.

•

In the double rod system in the following figure (b), the cement slurry is injected at a high velocity sheathed in a cone of air at an equally high velocity to erode and mix the soil well.

• The triple rod system in the following figure (c), uses high-pressure water shielded in a cone of air to erode the soil.

FIG.2: JET GROUTING

3.4 SAND DRAINS

• The use of sand drains is another way to accelerate the consolidation settlement of soft, nor- mally consolidated clay layers and achieve precompression before the construction of a desired foundation. • Sand drains are constructed by drilling holes through the clay layer(s) in the field at regular intervals. • This can be achieved by several means, such as (a) rotary drilling and then backfilling with sand; (b) drilling by continuous-flight auger with a hollow stem and backfilling with sand (through the hollow steam); and (c) driving hollow steel piles. • . Following figure 3 shows a schematic diagram of sand drains.

FIG.3: SAND DRAINS

3.5 SOIL NAILING

• Soil nailing is a construction technique used to reinforce soil to make it more stable. • It is used for slopes, excavations, retaining walls etc. to make it more stable. • In this technique, soil is reinforced with slender elements such as reinforcing bars which are called as nails. • It is just an alternate to retaining wall structures. • Soil nailing technique is used for slopes or excavations alongside highways, railway lines etc. Following figure 4 shows soil nailing in railway construction:

FIG.4 : SOIL NAILING DETAILS

TYPES OF SOIL NAILING:

There are various types of soil nailing techniques: 1 . Grouted Soil Nailing: In this type of soil nailing, the holes are drilled in walls or slope face and then nails are inserted in the predrilled holes. 2. Driven Nails: In this method, the nails are driven in the slope face during excavation. This method is very fast, but does not provide corrosion protection to the reinforcement steel or nails. 3. Self-drilling Soil Nail: In this method, the hollow bars are used. Hollow bars are drilled into the slope surface and grout is injected simultaneously during the drilling process. This method of soil nailing is faster than grouted nailing. 4. Jet Grouted Soil Nail: In this method, jets are used for eroding the soil for creating holes in the slope surface. Steel bars are then installed in this hole and grouted with concrete. 5. Launched Soil Nail: In this method of soil nailing, the steel bars are forced into the soil with very high speed using compressed air mechanism.

GEOGRID: • 3.6 A geogrid is geosynthetic material used to reinforce soils and similar materials. These are commonly used to reinforce retaining walls, as well as subbases or subsoils below roads or structures.

• Geogrids are commonly made of polymer materials, such as polyester, Polyvinyl alcohol, polyethylene or polyproylene. • The development of methods of preparing relatively rigid polymeric materials by tensile drawing,  in a sense "cold working," raised the possibility that such materials could be used in the reinforcement of soils for walls, steep slopes, roadway bases and foundation soils. • The key feature of all geogrids is that the openings between the adjacent sets of longitudinal and transverse ribs, called “apertures,” are large enough to allow for soil strike-through from one side of the geogrid to the other.

FIG.5: DIFFERENT TYPES OF GEOGRIDS.

• As discussed later, not only is rib strength important, but junction strength is also important.

3.7LIME STABILIZATION

• Admixtures are occasionally used to stabilize soils in the field— particularly finegrained soils. • The most common admixtures are lime, cement, and lime–fly ash. The main purposes of stabilizing the soil are to (a) modify the soil, (b) expedite construction, and (c) improve the strength and durability of the soil. • The types of lime commonly used to stabilize fine-grained soils are hydrated highcalcium lime Ca (OH) 2, calcitic quicklime (CaO), monohydrated dolomitic lime Ca (OH) 2 - MgO , and dolomitic quicklime. Lime Stabilization in the Field Lime stabilization in the field can be done in three ways. 1. The in situ material or the borrowed material can be mixed with the proper amount of lime at the site and then compacted after the addition of moisture. 2. The soil can be mixed with the proper amount of lime and water at a plant and then hauled back to the site for compaction. 3. Lime slurry can be pressure injected into the soil to a depth of 4 to 5 m.

3.8 CEMENT STABILIZATION

• Cement is being increasingly used as a stabilizing material for soil, particularly in the construction of highways and earth dams. • Cement can be used to stabilize sandy and clayey soils. • Cement stabilization is effective for clayey soils when the liquid limit is less than 45 to 50 and the plasticity index is less than about 25.

3.9 SAND COMPACTION PILES

• Sand compaction piles are similar to stone columns, and they can be used in marginal sites to improve stability, control liquefaction, and reduce the settlement of various structures. • Built in soft clay, these piles can significantly accelerate the pore water pressuredissipation process and hence the time for consolidation. • Sand compaction piles are constructed by driving a hollow mandrel with its bottom closed during driving. • . Sand is poured from the top of the mandrel and is compacted in steps by applying air pressure as the mandrel is withdrawn.